An expansion bolt is a fastener which is capable to grippingly secure itself in a hole formed on a wall or cement ground in advance with a threaded portion extending out of the hole to secure or engage other article by means of threading or nut. To be secured within the hole, the expansion bolt usually comprises a tubular member or hollow cylinder loosely and slidably fit over the bolt body. The tubular member has axially-extending slits formed on one end thereof and, corresponding to the slitted end of the tubular member, the bolt has an un-threaded end which is expanded and flared outward so as to be larger than the tubular member in diameter to provide an inclined camming surface which stops the sliding of the tubular member relative to the bolt body toward the expanded end of the bolt and serves to expand the slitted end of the tubular member when the tubular member is forced to slide over the expanded end of the bolt, which expansion of the slitted end of the tubular member allows the slitted portions of the tubular member to be in tight contact engagement with the inner side wall of the mounting hole so as to grippingly hold the bolt within the hole by friction therebetween.
An example of conventional expansion bolts is shown in FIGS. 1 and 2 of the accompanying drawings. The conventional expansion bolt, generally designated with the reference numeral 10, comprises an elongated bolt body 12 which has a threaded section 14 formed one end thereof and an un-threaded section 16 formed on an opposite end. The un-threaded section 16 comprises an expanded and flared end 18 which has an inclined circumferential camming surface 19. An expansion cylinder or tubular member 20 which has an inside diameter slightly larger than the bolt body 12 but smaller than the expanded end 18 is slidably and loosely fit over the bolt body 12 and may be secured thereon by a collar nut 22 to have an end 24 which has a plurality of axial slits 25 formed thereon abut against the inclined camming surface 19.
As shown in FIG. 2, to use the expansion bolt 10, a mounting hole 26 having an inside diameter slightly larger than the overall outside diameter of the expansion bolt 10 is formed by any known means, such as drilling, in advance in a wall or a solid fixture 28. By inserting the expansion bolt 10 into the hole 26 in such a manner to have the expanded end 18 located within the hole 26 and the threaded section 14 at least partially extending out of the hole 26, and by screwing the nut 22 inward to force the slitted end 24 of the expansion cylinder 20 to slide over the expanded end 18 of the bolt body 12, the slitted end 24 is expanded and flares outward by the camming action of the inclined camming surface 19 of the expanded end 18 so as to have the tips thereof forcibly engage the inside diameter of the mounting hole 26 to grippingly hold the expansion bolt 10 inside the hole 26. Articles or mechanical parts (not shown) to be secured to the wall or the fixture 28 can then be tightened on the threaded section 14 of the expansion bolt 10 which extends out of the mounting hole 26 and the expansion cylinder 20 by means of nut or threading.
Expansion bolt of this kind works well in securing articles to a fixture or supporting a great load on a wall. However, as can be seen in FIG. 2, if a great axial drawing force is applied to the expansion bolt 10, it is possible for the expansion bolt 10 to be forcibly withdrawn out of the hole 26 due to the fact that the inclining direction of the expanded end 18 of the bolt body 12 and thus the slitted end 24 of the expansion cylinder 20 which is expanded by the expanded end 18 of the bolt body 12 is in favor of the withdrawal.
It is therefore desirable to provide an expansion bolt structure which has an expansion cylinder to be expanded in such a manner to have the inclination of the slitted end oriented against withdrawal of the expansion bolt out of the hole in which it is secured.